In the present context an enclosed space is to refer, quite generally, to a zone which are sealed, largely or entirely, by a structural enclosure. Such a zone may be, for instance, a closed building, a closed container, or an area which is substantially enclosed within a building.
In industry, technical equipment often is housed in sealed spaces. When an operational disturbance or an accident occurs it may happen that a substance escapes which is contained, for example, in a pipeline system of a plant or in vessels associated with such plant or equipment. This may happen because a rupture or leak has formed. If the substance leaking out is such as to react chemically or physically with the atmosphere surrounding the plant, this is a potential risk for the plant. If the substance is an inflammable, volatile liquid, like benzene, alcohol, or the like, or an inflammable gas, like methane, hydrogen, or the like, the danger that a normal or explosion-type combustion will occur grows with the length of time such substance issues from the plant and has time to intermix, for instance, with the oxygen of the air.
If the substance so leaking is a chemically aggressive gas, there is a risk that even elements or parts of the plant at a great distance from the leak will be attacked or endangered if the gas spreads at sufficiently high concentration.
If the gas or gas mixture released by the chemical engineering plant or having formed subsequently, is a poisonous substance an essential gain in safety may be achieved if the latter can be confined to a limited area of the plant, thereby avoiding or at least restricting or delaying its release through such means as ventilation ducts, skylights, doors, and hallways, or the like, in order that protective measures devised for disaster control may be prepared and carried through successfully.
As an example of a conceivable accident in the chemical industry, reference may be made to the release of chlorine gas from a container or process circuit. This poisonous gas is especially dangerous when released close to the ground. If no more than minor heating occurs, such as by a fire breaking out at the same time, this gas may escape readily in an upward or lateral direction from a building and then cool down rapidly to such an extent that it will spread disastrously in the surroundings near to the ground. This may pose a threat to entire towns, particularly if meteorological conditions of inversion and unfavorable wind conditions prevail so that protective measures devised for disaster control, such as the evacuation of the population, cannot be carried out effectively. On the other hand, it is conceivable that the gas in question, which is relatively heavy at normal temperatures, will flow out in a downward or lateral direction into lower zones, for example through passages and gates although it was initially released in an upper region. It is possible as well that this gas will pass through ventilation ducts, even if the air circulating equipment is shut off, thus reaching zones which should remain accessible to afford an opportunity of controlling the accident. In any such case even reduced or delayed release would amount to an effective protective measure.
Similar, yet specific dangers threaten with nuclear power plants featuring light water reactors. Light water reactors comprise a primary coolant system in which "ordinary" water is used as the coolant.
In the case of light water reactors, allowance is made for the occurrence of serious accidents by way of technical safety measures providing for automatic shut-down of the light water reactor and switching on additional specific auxiliary means.
A rupture, or at least a leakage, occurring in a primary coolant line is called a loss of coolant accident and classified as a serious accident. When such loss of coolant accidents occur, for instance, molecular hydrogen may be formed within a short period of time by a metal-water reaction in the core area.
The explosiveness of an air/hydrogen mixture highly depends on the concentration of the reaction partners present. If, in such event, the proportion of air and of the oxygen available in the air reach a sufficiently high concentration, the high proportion of hydrogen will cause the formation of a gas mixture which may very easily be ignited. However, in the case of a combustion or explosion the effect may be of limited intensity if the amount of air or of the oxygen it contains which have become mixed with the hydrogen is insufficient or only partly sufficient to react with all of the hydrogen present. If an intermixing under such circumstances can be limited or avoided from the very beginning by not supplying further air, i.e. additional oxygen, to the place of the high hydrogen concentration, this will clearly reduce the hazardousness of the accident in question. Gaining but a few hours of time will play an essential role, all the more so as the short-lived fission products then will already have become decomposed to a large extent by natural decay.
It was suggested to fill the safety containments of a reactor with an inert gas during operation so as to prevent any inflammation or explosion of a substance reacting with the oxygen of air. However, this involves considerable disadvantages in normal operation and obviously cannot be realized with all plants.